1. Field of the Invention
The present invention relates to a crystalline polymer microporous membrane which has high filtration efficiency and is used for precise filtration of gases, liquids and the like, to a method for producing the crystalline polymer microporous membrane, and to a filtration filter.
2. Description of the Related Art
Microporous membranes have long since been known and widely used for filtration filters, etc. As such microporous membranes, there are, for example, a microporous membrane using cellulose ester as a material thereof (see U.S. Pat. Nos. 1,421,341, 3,133,132, and 2,944,017), a microporous membrane using aliphatic polyamide as a material thereof (see U.S. Pat. Nos. 2,783,894, 3,408,315, 4,340,479, 4,340,480, and 4,450,126, German Patent No. 3,138,525, Japanese Patent Application Laid-Open (JP-A) No. 58-37842), a microporous membrane using polyfluorocarbon as a material thereof (see U.S. Pat. Nos. 4,196,070, and 4,340,482, JP-A Nos. 55-99934, and 58-91732), a microporous membrane using polypropylene as a material thereof (see West German Patent No. 3,003,400), and the like.
These microporous membranes are used for filtration and sterilization of washing water for use in the electronics industries, water for medical use, water for pharmaceutical production processes and water for use in the food industry. In recent years, the applications of and amount for using microporous membranes have increased, and microporous membranes have attracted great attention because of their high reliability in trapping particles. Among these, microporous membranes made of crystalline polymers are superior in chemical resistance, and in particular, microporous membranes produced by using polytetrafluoroethylene (PTEF) as a raw material are superior in both heat resistance and chemical resistance. Therefore, demands for such microporous membranes have been rapidly growing.
Generally speaking, microporous membranes have a low filtration flow rate (i.e., a short lifetime) per unit area. In the case where the microporous membranes are used for industrial purposes, it is necessary to align many filtering units to increase the membrane areas. For this reason, a reduction in the cost for the filtering process is appreciated, and thus an extension of the filtering lifetime is desired. To this end, there are various proposals for a microporous membrane effective for preventing or slowing down reductions in flow rate due to clogging, such as an asymmetric membrane in which pore diameters are gradually reduced from the inlet side to the outlet side.
Moreover, another proposal is a microporous membrane of a crystalline polymer, which has a larger average pore diameter on a surface of the film than that on the back surface thereof, and has the pores whose average diameter continuously changes from the surface to the back surface (see JP-A No. 2007-332342). According to this proposal, fine particles are efficiently captured by the filter and the lifetime of the filter is improved, by performing filtration using, as the inlet side, the plane (i.e. the surface) having the larger average pore diameter.
However, there is still a problem such that no effective method has been proposed as a method for hydrophilizing a crystalline polymer microporous membrane having asymmetric pores.
The following are known as a hydrophilization method of a crystalline polymer microporous membrane: a hydrophilization method by applying an ultraviolet laser beam and ArF laser beam (for example, see JP-A No. 11-106553); and a hydrophilization method by performing a chemical etching using a sodium metal-naphthalene complex (for example JP-A No. 2007-154153). However, in the case of the crystalline polymer microporous membrane having an asymmetric pore structure, the membrane has a heated surface, unheated surface, and an inner part in between these surfaces, and the polymerization degree of the crystalline polymer varies in each area. The aforementioned hydrophilization methods cannot uniformly hydrophilize the entire membrane of such structure, and a hydrophilization process needs to be performed on each area separately depending on the polymerization degree thereof so as to uniformly hydrophilize the entire membrane, and the requirement for these separate hydrophilization processes results in low efficiency. Moreover, the membranes hydrophilized by these methods do not have sufficient hydrophilicity, and the filtration flow rate and lifetime thereof are also insufficient.
In addition, the hydrophilization method by applying an ultraviolet laser beam and ArF laser beam has a problem such that the radiation of the ultraviolet laser beam and ArF laser beam may damage the membrane, and thus the strength of the membrane may be decreased.
Also known is a method for improving hydrophilicity of a porous sheet formed of a fluororesin by covering the porous sheet with a water-insoluble hydrophilic material (for example, see JP-A No. 03-86206).
However, this method cannot improve the filtration flow rate and lifetime of the porous sheet formed of the fluororesin.